Various types of electric air heating devices using PTC (“positive temperature coefficient”) elements are known. A PTC element has a given resistivity at any given temperature, and the resistivity of the PTC element rises or falls with its temperature. In particular, the PTC element's resistivity rises exponentially once its temperature is increased over a certain temperature. Accordingly, once the PTC element's temperature is high enough, the resistivity of the PTC element becomes sufficiently high that the flow of current therethrough is nearly stopped. Because of this property, PTC elements have the beneficial characteristic of being self-limiting, thereby reducing the risk that an electric heater which includes a PTC element may cause a fire. However, in the prior art, PTC elements have been used primarily as sensors, to severely limit current when necessary for safety.
In addition, a heater with a heating element including one or more PTC elements which produces a specified output for a specified airflow is known. However, this prior art device does not provide for proportionate (i.e., variable) control of the heating element. Instead, this device produces a preselected power output for a preselected airflow when activated, i.e., the control is fixed because the heater can only be activated or de-activated, and if activated, only a certain output is provided thereby. Repeatedly turning this prior art heater on and off in response to signals from a thermostat tends to create significant changes in the ambient temperature, i.e., the typical thermostat does not signal for more heat until room temperature is relatively far below the setpoint temperature. Also, the typical thermostat does not stop a heater from operating until the setpoint temperature is exceeded, generally to an extent which is noticeable by those in the room.
There is therefore a need for an electric heating device which overcomes or mitigates one or more of the defects of the prior art.